High-output motors are used in hybrid cars, electric cars, and the like. A large electric current flows through the connector terminals of high-output motors or the like having a large conduction current, and thus the heat emission increases. The size of the connector terminals also increases in accordance with the current capacity, and therefore an insertion force required to insert them increases and the damage to the surface of the terminals at the time of insertion also increases. As the number of instances of the insertion and removal for maintenance of this type of connector terminals for large currents increases, there is the need for a connector terminal having heat resistance and abrasion resistance.
Conventionally, in general, a connector terminal obtained by plating a surface of a base material such as copper or a copper alloy with tin or the like has been used as a connector terminal that connects electric parts of a car or the like. However, in the case where the conventional tin plated terminal is used with such a large electric current, the conventional tin plated terminal does not have sufficient heat resistance. In view of this, there are cases where a silver plated terminal is used instead of the tin plated terminal as a connector terminal in which a large electric current is used. Silver has a low electric resistance value, keeps a temperature increase at the time of conduction low, has a high melting point, and achieves high heat resistance. Also, silver plating achieves very high corrosion resistance.
However, silver has properties of crystal particles easily coarsening due to recrystallization, and if a terminal obtained by performing silver plating is used in a high temperature environment, its hardness decreases due to the growth of crystal particles. Accordingly, problems such as an increase in the force required for inserting the terminal and an increase in its frictional coefficient arise.
In view of this, as shown in Patent Document 1, the inventor of the invention described in the present disclosure achieved a decrease in frictional coefficient of an electric contact by forming a layer structure in which the surface of a hard silver-tin alloy layer is coated with a soft silver coating layer in an electric contact of a connector terminal. The frictional coefficient can be reduced and softening is unlikely to occur even at a high temperature due to the hardness of the silver-tin alloy layer. Moreover, due to the fact that this silver-tin alloy layer is not exposed at the outermost surface and is coated with a silver coating layer that is relatively unlikely to oxidize, compared to the case where the silver-tin alloy layer is exposed at the outermost surface, an increase in the contact resistance resulting from the formation of tin oxides at high temperature can also be suppressed.